Animation Player not applying cubic keyframes correctly in 4.3dev2

[zolrath]

Tested versions

Reproducible in 4.3dev2
Not reproducible in 4.3dev1
Not reproducible in 4.2

System information

Windows 11 - Godot 4.3dev2 - Compatibility

Issue description

I've got an AnimationPlayer from a 4.2 project animating the rotation of a PanelContainer at the root of my scene.

When opening my 4.2 project in 4.3dev2, the animation no longer works correctly and the first/last frame with 0 degrees rotation are left at odd rotations.
 

4.2 and 4.3dev2 Comparison

4.2	4.3dev2

Adding an extra frame to end

If I add an additional 0 degrees frame at the end, both the last and second to last frames properly show 0 degrees.

Extra Frame 1	Extra Frame 2

Steps to reproduce

1. Create an Animation Player that rotates a panel container, 0.3 seconds long as shown above.
2. IMPORTANT: Use cubic interpolation, linear will not produce the error.
3. The rotation no longer matches the keyframe value.

I've copied over the offending panel into a MRP, along with a scene recreated in 4.3 using linear interpolation that doesn't seem to have the issue.

Minimal reproduction project (MRP)

AnimationPlayerBug.zip

I've included two scenes.
made_in_4_2_broken.tscn has the scene that showed the original issue reduced to its base parts.
made_in_4_3_seems_fine.tscn shows the same setup but with linear interpolation.

[Calinou]

Can you test to see if it works in 4.3.dev1?

[zolrath]

Seems to be fine in 4.3dev1

[bs-mwoerner]

Might be caused by [e538ce3]'s changes to cubic interpolation.

Before:

After:

[TokageItLab]

Sorry, I made a mistake in the argument there. Please test #87772 if you can.